1. Field of the Invention
The present invention relates generally to fluorescent lamps and more particularly to a low pressure mercury vapor discharge fluorescent lamp having a rare earth phosphor coating of multiple layers.
2. Description of Related Art
With regard to low pressure mercury vapor fluorescent lamps, it is known to use straight or cylindrical glass tubes and bent or non-straight glass tubes in the final lamp configuration. An example of the latter is a compact fluorescent lamp made with cylindrical straight tubing which is bent in manufacturing. With respect to lamps with non-straight tubes or glass envelopes, such lamps can be coated with phosphor before or after forming or bending of the glass tubing. If the forming is completed before the phosphor is coated on the inner surface of the tubing via a suspension, the suspension may not drain completely. In some configurations phosphor will settle preferentially to the bottom of a bend during draining. Even when drained by being rotated or shaken or otherwise moved in a complex fashion or by applying air pressure, the coating is often very non-uniform.
One solution to this problem is to first complete the coating of the phosphor on the straight tubing and then form the tubing to its final lamp configuration. However, this has been done with relatively thick single layers or coatings of phosphor (the single layer being of sufficient thickness to absorb substantially all the UV generated by the arc), but this single layer of phosphor tends to flake or partially come off in sections where the tube is formed or bent around a radius of curvature. What is needed is a procedure or approach which will result in the phosphor coating adhering better and not flaking or coming off in sections where the tube is bent or formed, during the forming procedure.